In U.S. Pat. 4,777,338, the inventor in common to the subject invention describes a device for producing spark perforation of synthetic plastic films. The perforation is performed by applying electrodes to opposite sides of the film as the film is passed through a water bath. The apparatus disclosed in this patent has certain limitations when operated as a perforation machine on a commercial scale.
The limitations of the perforation device disclosed in U.S. Pat. No. 4,777,338 arise from the behaviour of the water bath. While the water bath makes it possible to produce closely spaced perforations in a synthetic plastic film, it has been found difficult to maintain uniform small hole diameters. This problem arises because the water bath can vary in electrical conductivity and the charge lost from the electrodes due to conductive leakage will also vary. Furthermore, charge leakage through the large amount of water surrounding the electrodes makes the perforation process inefficient, because rather large amounts of electrical pulse energy are needed to overcome the charge leakage. The high electrical pulse energy, in turn, causes the electrode rollers to wear rapidly as they are small and have a small contact area with the film. The wear on the rollers results in a wide contact band and erratic perforation of the film. Contamination in the water bath is also difficult to control because the water bath has no controlled flow pattern and as a result contaminants can accumulate in the region of the electrodes. Another shortcoming in the prior art apparatus is the marking of the film as it is pulled through bath pressed between the roller electrodes and the ground electrode (i.e. the bottom surface of the bath).
In view of the foregoing, there remains a need for a new perforation apparatus suitable for commercial use which overcomes the shortcomings of the prior device.